This invention relates generally to tubular solar collectors and more particularly to a reflector for such a collector.
In the July 1976 issue of Applied Optics (Vol. 15, No. 7) Ari Rabl described a reflector which he suggested would give maximal concentration for a cylindrical absorber. When the absorber (which normally has a coating on its surface having high absorptivity and low emissivity) is contained in a glass cylinder or shroud, but spaced from the shroud, the equations Rabl uses to define the shape of the reflector cannot be directly applied for reasons which will be given later. These equations do serve as a starting point, however, and are given below using somewhat different notation.
The first equation is an involute of the circular cylindrical absorber surface and begins at the negative y axis (which is denoted as the angle .theta.=0). In accordance with convention, when considering solar collectors which are symmetrical about a center line, one half is dealt with, normally the right half. The involute continues going counterclockwise to the point where .theta.=.pi./2+.theta..sub.a. As is well known in the art, a concentrating reflector of the type here being considered has a field of view, and .theta..sub.a is the one half field of view. The field of view is that angle, measured from the vertical over which the reflector will reflect all solar energy rays passing between the opposite edges of the reflector (its aperture) onto the absorber. Thus, on a clear day, when the sun first rises above the horizon and continuing up to the time when the sun comes within the field of view, the reflector will not reflect all the solar energy rays passing through the aperture onto the absorber.
Rabl's disclosure illustrates but does not describe how "p" in the equations is used. For each .theta., a radius is drawn from the center of the absorber to the surface at the angle .theta., and then a line is drawn perpendicular to the radius a distance p down. The end points of these lines define the reflector surface.
The first equation is: EQU p=r.theta..
The second equation is: ##EQU1## In these equations: p=the perpendicular distance from the reflector to the absorber,
r=the radius of the absorber, PA1 .theta.=the angle measured counterclockwise from the negative y axis, and PA1 .theta..sub.a =the 1/2 field of view.
The second equation begins where the first one ends, at .theta.=.pi.2+.theta..sub.a. Rabl gives an end point of 3.pi./2-.theta..sub.a for this equation, but the present invention makes no use of it because an earlier termination is used.